BETWEEN DIISOPROPYLBENZENE DIHYDROPEROXIDE A N D IRON(II) 3273 July 20, 1036 THEREACTION
[CONTRIBUTION FROM THE RESEARCH AND DEVELOPMENT DIVISION,POLYMER CORPOUTION LTD.]
The Reaction between Diisopropylbenzene Dihydroperoxide and Iron(I1) or Some of its Complexes in Aqueous Solutions of Vinyl Compounds1 BY R. J. ORR AND H. LEVERNEWILLIAMS RECEIVED JANUARY 16, 1956
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The rate constant for the reaction: HOOC( CHa)zCeH4C(CHs)200H Fe(I1) + HOOC(CH~),CEH~C(CHI)~O Fe(II1) is k = 3 X 1012 l./mole/sec. a t pH 4.2. The free radical reacts with iron(I1) (at a rate )to), monomer ( h i ) and hydroperoxide (kip). E l i - El, and Ali/A1, were measured for the reaction in the presence of some watersoluble monomers with the following results: in acrylonitrile solution Eli - El, = -8.6 kcal./mole, and AIi/Al, = 3.1 X 10-6; in methyl acrylate solution E l i - E,, = -20 kcal./mole and Ali/A1, = 9.1 X 10-16; and in methyl methacrylate solution Eli - El, = -15 kcal./mole and A i i / A l o = 1.1 X lo-". klp/kl, is 10 a t 40' and 4 a t 25', where kl, is rate of reaction of radical with hydroperoxide. The free radical, after reacting with the monomer, yields a polymeric hydroperoxide which will react with iron(II), as long as the polymer molecule remains in aqueous solution. Vv'hen the iron(I1) is complexed with triethylenetetramine, a reaction occurs similar to that observed with monohydroperoxides.z The rate constant I./mole/sec. a t has been measured a t different pH; k = 1O1o e - l 0 J W R r l./mole/sec. a t p H 10.7 and k = 4 x loc8 e-10,700/RT pH 11.6. This reaction seems less sensitive to change of pH than was the case with monohydroperoxides. There was no necessity in interpreting the data t o consider the reaction between iron( 11) complex and polymeric hydroperoxide.
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e-1433D0/RT
Introduction This study is one of a series designed to give some idea of the role of hydroperoxide initiators in emulsion polymerization. The study has previously been confined to monohydroperoxides in the cumene and cyclohexane series in which the reactions with iron(II)3-6 and with the iron(I1)polyethylenepolyamine complexes2J were reported. Later work reported the tendency of the cumyloxy radicals to react with monomer.8 When a sample of p-diisopropylbenzene dihydroperoxide became available, the present study seemed pertinent. Experimental Methods The dihydroperoxide (DHP) was obtained from Hercules Powder Co. either in 20y0 acetone solution or as a white powder of 8070 hydroperoxide content. By evaporating the acetone and extracting the solids with benzene, the purity was raised to about 90%. This treatment should remove monohydroperoxides. Most of the remaining impurity was probably the alcohol resulting from thermal decomposition of the dihydroperoxide. Purification of the other compounds used, as well as the analysis for iron( 11), has been described p r e ~ i o u s l y . ~ ~ eI ~n 7 these same sources may be found descriptions of the methods used in performing an experiment, either using iron(II), or its polyethylenepolyamine complex as the reductant. To make a stock solution of the DHP, methanol was used to promote solubilizing so that the reaction occurred in an aqueous medium containing 0.5 to 2% methanol. Two methods of removing samples for analysis were used, depending on the time interval between samples permitted by the reaction velocity. For the slower reactions, samples were removed by pipet, permitting a standard volume to be used. For the more rapid reactions, the sample was blown into a graduated centrifuge tube containing a,a'-bipyridine which complexed the iron( 11) to an unreactive form, giving a color suitable for iron a n a l y ~ i s . ~The sample volume was measured after addition to the a,"-bipyridine solution. I t was possible to obtain samples a t frequent intervals in this manner. D H P is susceptible to polarographic analysis as demon, (1) Presented i n part before t h e Division of Physical and Inorganic Chemistry, American Chemical Society, Minneapolis, Minnesota, September, 1955. ( 2 ) R.J. Orr and H. L. Williams, Disc. Faraday Soc., 1 4 , 170 (1953). (3) J . W. L. Fordham and H. L. Williams, THIS JOURNAL, 79, 1634 (1951). (4) J. W. L. Fordham and H. L. Williams, i b i d . , 71, 4465 (1950). (5) R. J. Orr and H. L. Williams, Can. J . Chem.. 8 0 , 985 (1952). (6) R. J. Orr and H. L. Williams, J . Phrs. Chem., 67, 925 (1953). (7) R. J. Orr and H. L. Williams, Disc. Faraday Soc., 14, 170 (1953). ( 8 ) R. J. Orr and H. L. Williams, THIS JOURNAL, 77, 3715 (1955). (9) L. H. Jackson, I n d . Eng. Chem., A n a l . E d . . 10, 302 (1938).
strated by the polarogram in Fig. 1. Since only relative concentrations were required, no absolute calibration was made between [DHP] and I d . I d was measured a t -1.0 volt vs. a saturated calomel electrode.
Theoretical (a). Primary Rate Constants in the Presence of Excess Monomer.-By analogy with previous mechanisms postulated for iron(I1) and hydroperoxide reactions, the following mechanism should be written when monomer was present in such amount as to reduce [RO'],, to zero.
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ki HOOC( CH&C6H4C(CHs)200H Fe( 11) + Fe(II1) HOHOOC(CHI)ZCEH~C(CH~)~O.
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kl i
HOOC(CH~)ZCBHIC(CH~)~O' i- nM .--f HOOC( CH~)ZCSHIC( CH,),Ohtn'
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kz
'M,OC(CH~)~C.SH~C(CH~)~OOH Fe(I1) ----f Fe(II1) f HO- 4- 'M,0C(CHo)lCsHIC(CH3)z0.
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k2?
'M,OC( CHI)ZCBHX( CH3)20' nM 'MnOC( CH~)ZCBH~C( CH3)zOMu.
The type of reaction outlined can be treated kinetically if [Fe(II)]o = 2[HP]o, by the method outlined in the work of Frost and Schwemer.'O (b). Primary Rate Constants in the Absence of Monomer.-In the absence of monomer, conditions may be achieved where the oxidation of iron(I1) constitutes the only radical consuming reaction. The initial decomposition is identical to that shown above which is characterized by k,. The radical consumption step is
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klo
H O O C ( C H ~ ) ~ C , I H ~ C ( C H ~Fe(I1) )~~' + HOOC(CHI)ZCEH,C(CH~)ZO- Fe(II1)
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In the second step a novel species of hydroperoxide will be formed, which will lead to a novel type of radical
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ka HOOC(CHa)2C&LC(CH8)0zFe(I1) + ' O C ( C H , ) ~ C ~ H I C ( C H ~ ) Z Fe(II1) O-
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This radical will be consumed by the reaction (10) A. A. Frost and W. C. Schwemer, T R I SJ O U R N A L74, , 1268 (1952)
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